Known methods of treating metal surfaces to improve adhesion and corrosion resistance of the painted metal surfaces make use of two general definable classes of chemistries. The first class is based on the traditional conversion coating types of chemistries such as zinc phosphate, iron phosphates, chromium chromate, chromium phosphate, etc. The second class is based on more recent developments in the metal pretreatment industry and is characterized by what is now referred to as dried-in-place technology. The traditional conversion coating chemistries require rinsing of the metal substrate to remove applied conversion coating solution. Dried-in-place chemistries require that the applied solutions be dried on the metal substrate to which they are applied. Thus, they are described as dried-in-place, and the class of traditional conversion coatings have as their major drawback the requirement of rinsing, thereby consuming large volumes of water for rinsing.
An advantage of the dried-in-place class of chemicals is therefore obvious in that no rinsing is required. The traditional conversion coating class of pretreatments is characterized by steps or process stages that include:
1) Cleaning--by use of an alkaline or acid cleaning solution; PA1 2) Rinsing--to remove residual cleaning solution; PA1 3) Pretreatment--with the traditional conversion coating solution; PA1 4) Rinsing--to remove the residual pretreatment solution; and, optionally, PA1 5) a final rinse with a corrosion resistance enhancing material such as chromate. PA1 1) Clean--by use of an alkaline or acid cleaning solution; PA1 2) Rinse--to remove residual cleaning solution; and PA1 3) Application of the dried-in-place pretreatment solution.
The traditional coatings class of materials are therefore often referred to as five stage conversion coatings.
The second class of materials, the dried-in-place class, are typically referred to as three stage processes since their process steps are typically:
Another significant disadvantage of the traditional conversion coatings class is the fact that the rinse water is polluted and requires waste treatment. This adds significantly to the cost of the process because significant capital equipment and liability go along with waste handling and treatment.
There are also chemistries in both classes that have other major drawbacks. These particular chemistries are based on chromium compounds, which show toxicological properties and have been determined by the Environmental Protection Agency and by the Occupational Safety and Health Agency as a risk to the environment and health. Moreover, chemistries based on hexavalent chromium are classified as carcinogens by these agencies.
Attempts have been made to produce chromium-free dried-in-place coatings. In the case of aluminum extrusions, successful replacement of chromium was partially accomplished by use of a fluo acid and a carboxylic polymer, as disclosed in U.S. Pat. No. 4,191,596 to Dollman et al. The following formulas applied to clean aluminum extrusions at a concentration of 1% (by weight) of each in water will produce a coating that falls under this technology:
TABLE 1 ______________________________________ Compound 1 % Wt. Compound 2 % Wt. ______________________________________ Poly(acrylic acid) 5.0 Hydrofluozirconic Acid 7.4 Water 95.0 Hydrofluoric Acid 0.3 Water 92.3 ______________________________________
Aluminum samples treated by such a composition have acceptable paint adhesive properties in the dry state. The ability to pass other requirements of the American Architectural Manufacturer's Association (AAMA) specifications, however, are somewhat questionable. In some samples tested, the twenty minute cross-hatch boiling water test had variable results, and the 1,000 hour test in 100% relative humidity is inconsistent. In addition, neutral salt spray corrosion resistance is not good.
Another method has been introduced for aluminum extrusions. In this method, the following polymer solution along with a fluo acid is applied to an aluminum surface:
TABLE 2 ______________________________________ Compound % Wt. ______________________________________ Poly(acrylic acid) 3.75 Ammonium Bifluoride 0.10 Fluosurfactant 0.04 Water 96.11 ______________________________________
Although this method does not require chromium, it does require different process steps and requires an increase in the number of process steps. Comparing this method with a traditional conversion coating method, the step of treating the metal surface with a conversion coating solution is replaced with an acid cleaning stage, then water rinse, followed by dried-in-place pre-treatment which requires a stainless steel or acid-resistant section. In this method, the following steps are required to treat the metal surface: (1) cleaning the metal surface with an alkaline or acid bath; (2) rinsing the cleaned metal surface with water; (3) cleaning the metal surface with an acid solution; (4) rinsing the metal surface with water; and (5) pretreating the rinsed metal surface with a modified chromium-free solution, such as in Table 2 above. Thus, this method can be referred to as a "five-step, chromium-free, dried-in-place method." As in all methods, these steps are followed by drying and then painting the metal surface.
Nonetheless, there remains a need for a chromium-free coating composition which is capable of improving paint adhesion and corrosion resistance by employing the more convenient, three step dried-in-place method. In addition, there remains a need for such a composition which can improve paint adhesion and corrosion resistance of a variety of metals, not exclusively aluminum and aluminum alloys.
Further, there is a need for a coating composition having a relatively high concentration of constituents to meet application requirements such as some reverse-roll coating systems. Also, from an operational standpoint, a single package system including both the inorganic components and organic components is desirable. Thus, a single package material showing no instability or gelation is desirable.
International Publication No. WO 92/07973 to Reghi discloses a chromium-free corrosion surface treatment which is an aqueous acidic liquid comprising water and: (A) fluozirconic acid; (B) dispersed silica; (C) a particular polymer; and (D) 1-propoxy-2-propanol. Because silica can be undesirably attacked in an acidic composition, such as the composition disclosed by this international publication, concentrates that include silica are usually stable for only a few days. Accordingly, there is still a need for a composition for treating metal surfaces which includes silica and is stable for more than a few days.